Unaudited Financial Information (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017
Regulatory Asset [Line Items]
Regulatory assets	$ 129,743	$ 176,798
Regulatory asset - future recoverable pension costs	7,100	9,400
Regulatory Liabilities [Line Items]
Regulatory liabilities	1,499,747	555,657
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	737,746	0
Regulatory cost of service reserve
Regulatory Liabilities [Line Items]
Regulatory liabilities	30,930	0
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	528,709	521,330
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	159,201	15,559
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities	12,827	12,827
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory liabilities	20,551	0
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	9,783	5,941
Pension and postretirement benefit costs
Regulatory Asset [Line Items]
Regulatory assets	17,546	26,826
Infrastructure mechanisms
Regulatory Asset [Line Items]
Regulatory assets	77,387	46,437
Deferred gas costs
Regulatory Asset [Line Items]
Regulatory assets	347	65,714
Recoverable loss on reacquired debt
Regulatory Asset [Line Items]
Regulatory assets	9,328	11,208
Deferred pipeline record collection costs
Regulatory Asset [Line Items]
Regulatory assets	16,963	11,692
APT annual adjustment mechanism
Regulatory Asset [Line Items]
Regulatory assets	0	2,160
Rate case costs
Regulatory Asset [Line Items]
Regulatory assets	3,041	2,629
Other
Regulatory Asset [Line Items]
Regulatory assets	5,131	$ 10,132
Other Current Liabilities | Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 4,200